Field of the Invention
The invention generally relates to Circuit Switch Fallback (CSFB) technology, and more particularly, to apparatuses and methods for reducing the setup time of a CSFB call.
Description of the Related Art
With growing demand for ubiquitous computing and networking, various wireless technologies have been developed, such as the Wireless Local Area Network (WLAN) technologies, including the Wireless Fidelity (WiFi) technology, Bluetooth technology, and the Zigbee technology, etc., and also, the cellular technologies, including the Global System for Mobile communications (GSM) technology, General Packet Radio Service (GPRS) technology, Enhanced Data rates for Global Evolution (EDGE) technology, Wideband Code Division Multiple Access (WCDMA) technology, Code Division Multiple Access 2000 (CDMA2000) technology, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) technology, Worldwide Interoperability for Microwave Access (WiMAX) technology, Long Term Evolution (LTE) technology, Time-Division LTE (TD-LTE) technology, and LTE-Advanced (LTE-A) technology, etc.
For user convenience and flexibility, most User Equipment (UE) nowadays supports more than one wireless technology. Using a UE supporting both of the GSM/GPRS/EDGE/WCDMA technology and the LTE technology as an example, it may selectively obtain wireless services using the GSM/GPRS/EDGE/WCDMA technology or the LTE technology. Generally, it selects an LTE network over a GSM/GPRS/EDGE/WCDMA network when wireless services are available from both the service networks, since the LTE network may be more likely to provide wireless services with higher bandwidth than the GSM/GPRS/EDGE/WCDMA network. Nonetheless, in some cases, a technique called Circuit-Switched Fallback (CSFB) may be employed for a UE which is already camped on an LTE network to switch to the GSM/GPRS/EDGE/WCDMA network for accessing, particularly, Circuit-Switched (CS) services, e.g., a CS call, instead. The CS call made utilizing the CSFB technique is generally referred to as a CSFB call.
However, the setup time of a CSFB call may be too long and cause bad user experience. For example, a UE operating in the LTE idle mode is required to establish a Radio Resource Control (RRC) connection to transmit an EXTENDED SERVICE REQUEST message, so that information of one or more target frequencies may be obtained from the LTE network, which indicates the UE to search for a cell on the target frequencies to make the CSFB call. Yet, there are situations where the RRC connection establishment may be delayed due to bad LTE cell quality, or there are situations where the cells on the target frequencies may not be able to provide fair signal quality for the UE to make the CSFB call successfully.